1. Field of the Invention
The present invention relates to a method of compiling a list of usable neighbour base stations measurable by a location measurement unit (LMU), to a serving measurement location centre for implementing the method and to a computer program.
2. Description of the Related Art
In a mobile communication network comprising a plurality of base stations, it is known to determine the location of a mobile station in the network in the following way. A mobile station MS receives signals transmitted via base stations and can determine the time difference between receptions. These are called observed time differences (OTD). The synchronisation difference between transmissions from the base stations is called the real time difference (RTD). If both RTD and OTD are known, a so-called geometrical time difference (GTD) can be determined from the equation:GTD=OTD−RTD  Equation 1
To determine the position of a mobile, take the example of three base stations, a location measurement unit and a mobile station. A serving mobile location centre (SMLC) in the network implements the location measurement technique in conjunction with the LMUs associated with the base stations. The LMU determines (or more precisely SMLC determines based on the measurements from the LMU) a first RTD and the mobile station identifies a first OTD between a first pair of base stations. Then, a second RTD is determined by the SMLC using the LMU measurements from a second pair of base stations, and the mobile station determines a second OTD between the second pair of base stations. A positional algorithm implemented at the SMLC takes the first set of RTD and OTD measurements and determines the first GTD using equation 1. This GTD, when taken with the geographical coordinates of the base stations of the first pair can be used to define a hyperbolic line on which the mobile resides. Similarly, the second set of RTD and OTD measurements is taken to determine a second GTD which, when taken with the geographic coordinates of the base stations of the second pair defines a second hyperbolic line. The location of the mobile can the be approximated by considering where the two hyperbolic lines intersect.
A first way of determining the RTD is for the LMU to measure the OTDs and then to calculate the RTD from equation 1 because the coordinates of the LMUs and the base stations are known (i.e. the GTD is known).
Another approach is for the LMUs to have a common accurate clock (such as GPS) to allow the arrival time of signals to be timestamped. This is sometimes called OAT (observed absolute time). When the distance between an LMU and an associated BTS is reduced from OAT, an estimate for the time when the signal was transmitted is obtained. This timestamp is called AT (absolute time). The RTD value for any pair of base stations can be simply calculated as the difference between the corresponding two absolute time (AT) values.
This calculation can be carried out either in the network itself, that is in the serving mobile location centre, or at the mobile. In the latter case, the RTD and the geographical coordinates of the base stations are transmitted to the mobile station.
Such a location measurement technique is managed by SMLC in conjunction with the location measurement units associated with the base stations. It will be appreciated that it is not necessary to have an LMU associated with each base station. Each LMU can measure any base station within its range over the air interface. Also, it has a physical or air interface connection to one base station to allow it to communicate with the SMLC. To achieve reliable measurements of geographic locations of mobile stations, it is important to know which base stations can usefully be used to provide the necessary measurement. In current networks supporting location based service, so-called neighbour (nbor) lists are held. These neighbour lists define for each target LMU a group of adjacent base stations that can be used to make measurements for determining the geographic location of a mobile station. Currently, such neighbour lists are manually created by experienced field engineers and depending on the network configuration can take several weeks. An experienced field engineer must basically determine which adjacent base stations to a target LMU can obtain accurate timing measurement. In order to determine this, a field engineer must determine if received signals from adjacent base stations suffer from multipath and/or interference. Firstly, an initial neighbour list can be created by a field engineer by analysing the network layout, for example from looking at the distance of the adjacent base stations to the target base station, antenna directions or width, linear side, topography and frequency plans.
Next, the neighbour list can be verified by analysing measured signals from adjacent LMUs to the target base station. Verification of this list can be carried out by analysing signal measurements collected by adjacent LMUs, by looking at signal strength, bit error rate and optimisation measurements, for example.
Not only is such a technique time consuming and labour intensive, but it also needs to be carried out each time there is a change in the network, for example frequency re-allocation or deployment of new equipment.
It is an object of the invention to enable a list of usable location measurement units to be compiled without reliance on manual activities and personal expertise.